A passive optical network (PON) is one system for providing network access over “the last mile.” A PON is a point-to-multipoint network comprised of an optical line terminal (OLT) at a central office, an optical distribution network (ODN), and a plurality of optical network units (ONUs) at the user premises. PONs may also comprise remote nodes (RNs) located between the OLTs and ONUs, for example, at the end of a road where multiple users reside. In recent years, time division multiplexing (TDM) passive optical networks (PONs), such as Gigabit PONS (GPONs) and Ethernet PONs (EPONs), have been deployed worldwide for multimedia applications. In TDM PONs, the total capacity is shared among multiple users using a time division multiple access (TDMA) scheme, so the average bandwidth for each user is limited to well below 100 megabits per second (Mbps) for GPONs and EPONs.
As user bandwidth demands have increased, ten gigabits per second (Gbps) (10 G) PONs (e.g., ten gigabit (XG)-PONs and 10 G EPONs) have also been standardized for next generation optical access. Wavelength division multiplexing (WDM) PONs are considered a very promising solution for future broadband access services. WDM PONs can provide high-speed links with dedicated bandwidth up to 10 G. In a WDM PON, each ONU is served by a dedicated wavelength channel to communicate with the central office or the OLT. However, the number of users who can be served by WDM PONs is typically limited to 64 or less due to limited wavelengths available for WDM PONs and a limited operation wavelength range for colorless ONUs. A hybrid approach combining TDM with WDM can support higher capacity so that an increased number of users can be served by a single OLT with sufficient bandwidth per user. TDM/WDM PONs present design and cost issues that must be addressed.
Previously, at least one TDM/WDM PON achieved colorless burst-mode transmission by employing a tunable optical transmitter as described in “40 Gbit/s λ-tunable stacked-WDM/TDM-PON using dynamic wavelength and bandwidth allocation,” Hirotaka Nakamura, Optical Society of America, 2011, which is incorporated by reference as if reproduced in its entirety. Colorless transmission may mean that transmission is not limited to any particular wavelength. Burst-mode transmission may mean that transmission occurs in bursts of relatively short periods of duration. Burst-mode transmission may be necessary in a TDMA scheme. In the PON described by Nakamura, multiple wavelengths corresponding to an arrayed waveguide grating (AWG) channel shared a single feeder fiber using a WDM scheme, and each wavelength was further shared among multiple users using a TDMA scheme. A tunable optical transmitter at the ONU provided the colorless transmission, but tunable lasers at ONUs may create cost concerns.
Alternatively, at least two TDM/WDM PONs achieved colorless burst-mode transmission by employing a seeded optical transmitter as described in “Demonstration and Field Trial of a Scalable Resilient Hybrid ngPON,” J. Prat, Optical Society of America, 2011 and “Dense WDM-PON based on Wavelength Locked Fabry-Perot Lasers,” Sang-Mook Lee, Optical Society of America, 2005, which are incorporated by reference as if reproduced in their entirety. The PONs described by Prat and Lee were essentially the same as the PON described by Nakamura, but employed a seeded optical transmitter instead of a tunable optical transmitter. A high-power broadband light source at the central office was necessary to provide a seed light, but high-power broadband light sources at central offices may provide cost and upstream performance concerns, namely Rayleigh backscattering from the seed light.
At least one WDM PON achieved colorless transmission by employing a self-seeded reflective semiconductor optical amplifier (RSOA) transmitter as described in “Stable self-seeding of R-SOAs for WDM-PONs,” Marco Presi, Optical Society of America, 2011, which is incorporated by reference as if reproduced in its entirety. In the PON described by Presi, multiple wavelengths corresponding to an AWG channel shared a single feeder fiber using a WDM scheme. Amplified spontaneous emission (ASE) noise from the RSOA transmitter was filtered by an AWG, reflected back by a Faraday rotator mirror (FRM), and seeded into the RSOA, thus providing colorless transmission. The use of a WDM scheme, but not an accompanying TDMA scheme, may not take full advantage of potential bandwidth because the dedicated bandwidth is wasted when user has no data to send.